Stockton (Utah): The US Army will have destroyed about 90 percent of its aging chemical weapons after it wraps up work this week in Utah, where it has kept its largest stockpile, a mix of toxins and blister and blood agents that accumulated through the Cold War, officials said on Thursday.

The Army's Deseret Chemical Depot in Utah planned to burn its last hard weapons in a furnace on Thursday. The projectiles contain mustard agent, which can produce painful skin blisters. The depot expects to complete the job by the weekend.

The US is part of an international treaty to rid the world of chemical weapons, a campaign taking place with spotty success around the globe. The goal was supposed to be accomplished by April 29 but will take years longer.

"Clearly, it's still a tremendous example of what the world can do," said Craig Williams, director of the US-based Chemical Weapons Working Group, an advocate for safe disposal.

"You've got 188 of 194 countries on the planet signing the treaty. It's an impressive effort, a great step forward for the safety of the world."

Chemical weapons were introduced into warfare during World War I, killing 90,000 troops on battlefields, according to the Organisation of for the Prohibition of Chemical Weapons in The Hague, Netherlands. More recently, the regime of former Iraqi dictator Saddam Hussein lobbed a chemical cocktail on a Kurdish town in 1988. That became the inspiration for the 1997 treaty to rid the world of chemical weapons.

As far as it is known, the US has never fired a chemical weapon in anger, although some consider the use of the defoliant Agent Orange during the Vietnam War a chemical attack, Williams said.

The US has acknowledged it will take as long as 2021 to finish destroying the final 10 percent of its chemical weapons at depots in Colorado and Kentucky. Russia is far behind in its effort, having destroyed only about 48 percent of a large cache of chemical weapons, according to the Organisation of for the Prohibition of Chemical Weapons.

An international tribunal voted last month to waive trade or other sanctions and instead subject the US and Russia to increasing pressure and inspections. Each country must submit plans by April 29 detailing how they will finish the job "in the shortest time possible."

Libya also is expected to miss the deadline. The recent uprising there interrupted that country's work and exposed more chemical weapons depots than were thought to exist, Williams said.

The Deseret Chemical Depot in Utah once contained 44 percent of the nation's supply of chemical agents. The depot didn't just hold obsolete US weapons. A supply of nerve agent seized from Nazi Germany at the end of World War II was destroyed only months ago.